1. Technical Field
The present disclosure relates to a variable gain amplifier and more particularly, to a feedback-type variable gain amplifier including a feedback circuit.
2. Discussion of Related Art
A variable gain amplifier is an amplifier that can have its gain controlled. A feedback-type variable gain amplifier is an amplifier that can have its gain controlled by an impedance of a feedback circuit.
A feedback-type variable gain amplifier is disclosed in U.S. Pat. No. 6,285,257 and Korean Patent No. 2003-0089067. An operation of a feedback-type variable gain amplifier is described in “A 3-10-GHz low-noise amplifier with wideband LC-ladder matching network” Aly Ismail et al. IEEE Journal of Solid-state Circuits, VOL. 39, NO. 12, December 2004.
Referring to the journal article, an input impedance of an amplifier is expressed as Formula 1 below, in which ZFB denotes an impedance of a feedback circuit, ZL denotes at impedance of a load circuit, and gm denotes a trans-conductance of a transistor.
                    Zi        =                              ZFB            +            ZL                                1            +                          gm              ·              ZL                                                          [                  Formula          ⁢                                          ⁢          1                ]            
According to Formula 1, if the feedback impedance ZFB is changed in order to control a gain of the amplifier the input impedance of the amplifier is also changed.
If the input impedance of the amplifier is changed, an impedance match with other circuit blocks coupled to the amplifier may be broken. In addition, if the input impedance of the amplifier is changed, a reflection ratio of an import signal is also changed.
A voltage gain AV of an amplifier is expressed as Formula 2.
                    AV        =                              VO            VIN                    =                                    -                              (                                  gm                  -                                      1                    /                    ZFB                                                  )                                                    (                                                1                  /                  ZFB                                +                                  1                  /                  ZL                                            )                                                          [                  Formula          ⁢                                          ⁢          2                ]            
According to Formula 2, the voltage gain AV of the amplifier may be controlled by using the impedance ZFB of the feedback circuit and the impedance ZL of the load circuit.
A voltage standing wave ratio (VSWR) is used to indicate a reflection amount of a signal at an input terminal in a circuit. If a reflection coefficient is denoted as Γ, the VSWR is expressed as Formula 3.
                    VSWR        =                              1            +                                        Γ                                                          1            -                                        Γ                                                                        [                  Formula          ⁢                                          ⁢          3                ]            
If the input impedance of the amplifier is changed, the reflection coefficient is changed. In addition, if the reflection coefficient is changed, the VSWR is also changed.
Therefore a feedback-type variable gain amplifier that can control a gain of an amplifier without changing the amplifier input impedance is required.